1. Field of the Invention
The present invention concerns a percussion sawing machine to saw stone blocks into slabs. As known, these machines essentially consist of a fixed part, acting as support, and of a movable part which comprises the oscillating blade frame.
The fixed part consists of a stout framework comprising four vertical steel columns, fixed at the bottom to a reinforced concrete foundation and connected at the top by four steel cross members. A large worm screw is positioned inside each vertical column and the four screws are caused to rotate simultaneously by suitable motor means. Each screw then carries, in a screw-and-nut relationship, a corresponding lead nut to which is fixed the movable part of the sawing machine.
The movable part essentially consists of a horizontal rectangular blade frame, into which is fixed a set of parallel and suitably tensioned saw blades, consisting of steel plates having a scarcely thick rectangular cross section. The dimensions of the blade frame are substantially equal to the inner dimensions of the fixed part of the sawing machine, into which said frame oscillates moving simultaneously downwards so as to perform the cutting of the block into slabs.
In the sawing technique making use of the aforedescribed machines, the steel blades of the blade frame simply have the function to apply the actual stone cutting medium, which consists of a thick slurry of water and abrasive material. Said slurry is constantly circulated over the stone block being cut by a suitable pump positioned into a tank provided under said stone block, wherein the slurry is collected. Different types of abrasive material are of course used, according to the kind of stone block being cut: sand has traditionally been used for sawing marbles, whereas a mixture of steel grit and lime is adopted for granites. The slurry is periodically or continuously reactivated by adding fresh abrasive material, so as to make up for that consumed in the cutting operation.
In all the sawing machines adopted at present, the oscillating movement of the blade frame is of the hunting type, obtained by fixing said frame to the fixed part of the machine by means of four rocking arms connected--freely rotatable by way of hinged joints--by one end to the four corners of said blade frame and by their opposite end to the previously cited lead nuts.
With this type of construction, during the alternate huntings of the blade frame each point of the single saw blades describes a circle arc trajectory, of radius equal to the length of the rocking arms and of maximum chord (commonly called "travel") equal to twice the length of the crank of the connecting rod-crank system imparting the alternate motion to the blade frame. The effective part of this travel, namely that in which the abrasive cutting action actually takes place, is obviously only that in which the blades are in contact with the stone block to be cut, being therefore--from a theoretical point of view--only the central point of the hunting travel. In the remaining part of their travel, the blades rise from the bottom of the respective channels cut into the stone block, allowing a further essential function of the sawing machine to be performed, namely the outlet from the cutting channels of the consumed slurry and the simultaneous feeding of fresh slurry required for the sawing operation.
In actual fact, the effective contact length between the blades and the block being cut is far longer and, taking also into account the partial deflection of the blades tensioning system while they bear onto said block, the average values of the contact length are equal to 30-35% of the travel, in sawing machines having rocking arms about 1 meter long. Said value--also known as abrasion ratio--directly answers for the sawing speed and thus for the productivity of the machine.
2. Description of the Prior Art
Numerous studies have been carried out since several years and different solutions have been proposed to rise said abrasion ratio beyond the aforespecified values, namely to increase the period of the abrasive action in respect of the period in which the blades are not in contact with the bottom of the cutting channels, allowing the slurry to be fed. The feeding of this latter should in fact be fast enough and be equally regular, even if the period in which the blades part from the bottom of the cutting channels were to be far shorter than in the case of conventional sawing machines.
The different solutions proposed by technique to supply sawing machines with an increased abrasion ratio can be divided into two categories, namely machines with a hunting travel and machines with a rectilinear travel. The first category comprises machines which, though adopting different constructive solutions, keep to the traditional arrangement of the blade frame suspended to rocking arms; the second category includes instead machines which slide or roll onto suitable supports, so as to travel at least partly on a straight line.
In the sawing machines of the first category, the most obvious solution has evidently been to increase the length of the rocking arms, so as to thus increase the radius of the hunting movement of the blades, flattening their trajectory, without modifying the arc shape of said trajectory. This system allows to increase the abrasion ratio to about 50%, but nevertheless brings the total height of the machine to unacceptable values, both for the overall dimensions involved and for the excessive flexibility thus acquired by the framework.
A more recent solution, always belonging to the first category of sawing machines, consists in replacing the conventional hinged joints by double-acting hinged joints. In this case, the rocking arms are not directly pivoted on the axis of the hinged joint, but onto an eccentric rotating inside the joint. This allows to vary the length of the rocking arm, during the hunting travel, from a minimum in correspondence of the central point of the travel, to a maximum in correspondence of the dead centres, thereby realizing the desired flattening of the blades trajectory. Also in this case, the abrasion ratio can be increased to about 50%, but this solution introduces in the sawing machine a rather delicate mechanical member, as the double-acting hinged joint, which is not apt to stand for long the high stresses involved. Thus, in addition to a higher initial cost, this solution also requires frequent operations of maintenance and repair.
Furthermore, a feature common to all the types of sawing machines belonging to the aforementioned first category, is that the blades arc trajectory is strictly tied to the dimensions of the structural parts of the machine, and particularly to those of the rocking arms. Said trajectory is thus calculated once for all when the machine is under production and can no longer be modified during the useful life of the machine, unless the same were to be totally restructured.
Concerning the sawing machines of the second category, it should be observed at once that none of them has found up to date an appreciable industrial application, though the first studies in this field date back to the start of the century. The DE-C-172 858, dating back to 1904, describes in fact a sawing machine of this type, wherein the blade frame slides onto four straight tracks, by way of wheels pivoted idle to the corners of said frame and laterally provided with pins cooperating with a guide rack. This document also teaches to vary the shape of the tracks, or that of the wheels, so as to obtain sliding trajectories of the blade frame wherein the blades part from the bottom of the cutting channels, both at the two dead centres of their travel and, possibly, in correspondence of a central point, thereby allowing an improved feeding of the abrasive slurry in the cutting area.
This second category also includes the FR-A-1 008 422, which describes a sawing machine with rectilinear travel, wherein the sliding takes place between a substantially flat support and a circular wheel, which can be respectively fixed onto the lead nuts movable on the screws and onto the blade frame, or viceversa. Also this document teaches the possibility to part the blades from the stone block being cut, at the opposite ends of the travel and, possibly, also in the centre, so as to allow feeding the abrasive slurry.
The sawing machine described in FR-A-1006643 is an example of another embodiment of a rectilinear travel sawing machine. In this case in fact the movement of the blade frame--in a different way from what it occurs in the two previous cited patents--it is obtained through a toothed wheel and rack. The sawing machine has also a vibration system of the cutting elements operated by compressed air or by cams, acting in the two directions perpendicular to the principal movement direction of the cutting elements themselves.
As specified heretofore, none of the above solutions has found a practical industrial application, in spite of offering--in theory--the possibility to realize sawing machines with an abrasion ratio positively higher than those according to prior art and, furthermore, apt to be programmed at will by simply replacing the sliding tracks. The reasons for said unsuccessful results are probably determined by two independent causes.
On one hand, the fact that the construction of this type of sawing machines is more complex and thus more costly than that of conventional sawing machines and, furthermore, that it is not easy--from the technical point of view--to realize a construction of the sliding and rolling members which would not suffer from the drawbacks determined by saw dust and sludge, met in the sawing of stone blocks. It should in fact not be forgotten that, even slight deposits of material onto the sliding surfaces, can considerably alter the wanted trajectory of the blade frame or even hamper the regular working of the sliding and rolling members.
On the other hand the fact that, possibly, the higher abrasion ratio obtainable--in theory--with sawing machines having a rectilinear travel path, could even--in practical application--not be effectively utilized, due to the requirement of feeding the cutting channels with fresh abrasive slurry. It does not seem in fact a mere coincidence that the first two above cited documents should provide for the possibility to lift the blades from the bottom of the cutting channels at the centre of the travel, just in order to fulfil said requirement. It is hence legitimate to think that the increase in the abrasion ratio--obtainable in practice with sawing machines with rectilinear travel--is not so significant as to make up for the financial burdens deriving from the more complex structure of said machines and from the fact that the market trend is totally in favour of sawing machines with hunting travel.